Venetian blinds are well known and typically include a head rail, a bottom rail, and a plurality of slats arranged between the headrail and the bottom rail. The slats are typically made from a variety of materials, such as metal, wood, plastic or other materials and supported by ladders.
Such blinds also typically include a tilt mechanism to enable the slats to move from a horizontal position to a nearly vertical position to open and close the blinds to affect the passage of light. As is also conventional with such systems, flexible line members or lift cords are coupled to the bottom rail, pass through the slats or adjacent the edges of the slats and into mechanisms within an upper headrail. The cords are employed to raise the bottom rail, accumulating individual slats as the bottom rail is raised. Because of gravity, the natural tenancy of the bottom rail and accumulated slat weight is to free fall. In many instances in the prior art, cord lock mechanisms are employed to lock the cord, thereby setting bottom rail, and the slats stacked thereon at a height determined by the user. Pleated and other types of shades also include a bottom rail and include similar raising and lowering lift cord members and a cord lock mechanism.
Blinds and shades that use a cord lock mechanism typically include a portion of the lift cord that extends from the cord lock and is external to the blind or shade. This external portion of the lift cord may pose a danger to small children or pets. A cordless blind, such as the one disclosed in U.S. Pat. No. 5,482,100 to Otto Kuhar eliminates the need for a cord lock and the resulting external portion of the lift cord. The cordless blind may employ a spring motor to assist in the balance of the bottom rail and accumulated window covering material.
Spring motors used in cordless blinds may comprise a flat ribbon of spring metal which is pre-stressed and coiled so as to have a natural or relaxed state in which the spring forms a tightly wound coil disposed on or in a spring storage or take up drum. The extended free end of the coil is attached to the hub of an output or spring drive drum onto which the spring is backwound by rotating the output drum in a direction to back or reverse wind the spring thereon. When the load to which the output drum is connected is released, the curling property of the spring causes it to rewind onto or into the storage drum toward its natural or relaxed state. Such spring motors as descried above can be of constant or variable force, depending upon the intended use of the motor. Other type of spring motors may also be used.
In connection with the use of such a spring motor and a venetian blind, as an example, a control drum or spool is coupled to one of the storage or output drum for rotation therewith. Depending on the number and location of the lift cords, the lift cords may be attached to a single spool or to two or more spools. The flexible member or lift cords are wound onto the spool in a direction which provides for the unwinding of the cords to rotate the spring output drum in the direction for winding the spring member thereon from the spring storage drum.
When the bottom rail is lowered, the cords unwind from the spool or spools thus driving the spring output drum to wind the spring member thereon. Upward displacement of the bottom rail toward the head rail from a lowered position results in the spring member rewinding on the spring storage drum to rotate the spring output drum and thus the spools in the direction to rewind the cords. In elevating and lowering a suspended load of the foregoing example type, which is too heavy to provide desire displacement characteristics in connection with the upward and downward movement of the bottom rail, and using a single spring motor, many times it is necessary to provide a larger spring motor or operate two or more spring motors in tandem.
If the spring motor has a spring force that varies such that the force increases as the bottom rail is moved toward the head rail, the spring force may balance the increased weight of the window covering that accumulates on the bottom rail as it is raised. However, if the spring force is a constant force then the spring force and weight of the bottom rail and accumulated window covering may not be in balance for the full range of positions of the bottom rail relative to the head rail.
If a spring motor is selected with a spring having a spring force sufficient to maintain the bottom rail and accumulated window covering in the raised position, the spring force may be excessive when the bottom rail is in the lowered position. As a result the bottom rail may creep upward until the cordless blind system is in balance. Creep being the movement of the bottom rail relative to the head rail away from the desired position as set by an operator of the blind.
Because of the difference in materials of the slats, the size of the blind, the number of slats in the blind, the weight of the parts plus the weight of the bottom rail, etc., the motor must have different characteristics and be designed for different loads. To avoid having to design a separate motor for each type or combination of blind components, a balance of forces, typically in friction forces, within the blind system is desired. Where the spring force is sufficient to maintain the bottom rail and accumulated window covering in the raised position, but to strong to allow the bottom rail to maintain its position in the lowered position, additional friction is required. However, the additional friction is only required in a single direction to avoid upward creep.
Thus there is a need for a cordless blind that will provide a balance of forces to avoid creep of the bottom rail. There is a further need for a friction applying system that will provide one-way tension to a cord in a blind system. There is a further need for a blind system that includes a mechanism for providing a resistant force on cord movement in one direction and a free-wheeling capability on the cord in another direction.
It would be desirable to provide a blind with or providing any one or more of these or other advantageous features.